Apparatus and method for removing carbon dioxide (co2) from the flue gas of a furnace after the energy conversion

ABSTRACT

Disclosed is a method for separating carbon dioxide from a flue gas from energy conversion, wherein the flue gas is first supplied to a membrane unit, comprising at least one membrane module having a CO 2 -selective membrane, with a portion of the CO 2  being separated in the module from the flue gas as an enriched CO 2  permeate. The flue gas thus depleted (retentate) is subsequently supplied to at least one additional CO 2  separating unit, in which a portion of the CO 2  is separated from the flue gas by an adsorbent or absorbent. Also disclosed is a device that is suitable for carrying out the aforementioned methods for separating CO 2  from a flue gas using a membrane unit, comprising at least one membrane module having a CO 2 -selective membrane and having an additional CO 2  separating unit, comprising an adsorbent or absorbent.

The invention relates to methods for reducing CO₂ emissions from the waste gases of combustion plants, and particularly from flue gases of energy conversion plants, using a combination of methods. The invention further relates to devices suited for carrying out these methods.

PRIOR ART

One of the most significant sources of increases in atmospheric carbon dioxide concentration is the combustion of fossil fuels in combustion plants with the goal of converting energy. Thus, attempts have been undertaken to separate CO₂ from the combustion of fossil fuels and thereafter store it, so as not to release it into the atmosphere. The reasons for these endeavors are the greenhouse effect and resulting global warming. At present, three basic approaches to separation of carbon dioxide are being pursued, which differ in the positioning of the separation step in the energy conversion process.

These approaches are CO₂ separation prior to energy conversion (pre-combustion), production of a flue gas rich in CO₂ by way of energy conversion in an enriched oxygen atmosphere (oxy-fuel), and CO₂ separation after energy conversion (post-combustion).

In the pre-combustion process, a fuel, such as coal for example, is freed of CO₂ even before the energy conversion. To this end, the fuel is gasified at a high temperature to form a synthesis gas made of carbon monoxide (CO) and hydrogen (H₂). In the oxy-fuel method, the fossil fuel is not burned in air, but ideally with pure oxygen.

As an end-of-pipe solution, the approach of CO₂ separation after energy conversion is advantageous in that the CO₂ separation step itself has little influence on the availability of the energy conversion plant. Moreover, it allows for retrofitting of existing plants. Depending on the type of power plant, type of fuel, and type of combustion of the energy conversion plant, the flue gas that is released into the atmosphere contains between 3 and 15% by volume CO₂.

A variety of methods can be employed to separate CO₂ from flue gas, for example chemical or physical absorption in absorption towers or in membrane contactors, or a CO₂-selective membrane.

CO₂ scrubbing by means of chemical absorption (CAS=chemical absorption system) is carried out as follows. Following flue gas scrubbing (for example, in the case of a coal-fired power station, consisting of nitrogen oxide reduction, dedusting and desulfurization), the flue gas is conducted through an absorbent, which is typically liquid. This absorbent generally contains amines. The CO₂ can first be chemically or physically bound in the scrubbing agent (absorption). Subsequently, the CO₂-loaded absorbent is heated, whereby the CO₂ is again separated from the amines (desorption). The released CO₂ is substantially pure and can subsequently be compressed, and optionally transported to a storage unit or for use. The amine-containing absorbent can be reused after the CO₂ has been released, and optionally after conditioning. For low to moderate CO₂ partial pressures, a degree of separation of up to 90% can thus be reached.

Regenerating the absorbent, however, is an energy-consuming process because, on one hand, the CO₂-loaded absorbent is heated and, on the other hand, the exothermic reaction, during the absorption, between the absorbent and the CO₂, is reversed by the supply of heat. The necessary heat, which is withdrawn from the power plant process, generally disadvantageously impacts the efficiency of the power plant.

In addition to amine solutions, other absorbents are also being tested at present. The scrubbing agent with the lowest energy expenditure will prevail.

As an alternative to chemical absorption scrubbing, separation by means of membrane contactors serving as the gas-liquid contact apparatus is presently being explored (MAS=membrane absorption system). Membrane contactors advantageously enable a dispersion-free mass transfer between two phases, and generally require only one tenth of the structural dimensions as compared to conventional chemical scrubbers.

In the membrane contactor, the gas and liquid flows are always separated from each other by the membrane surface. Multiple hollow membrane fibers are embedded in the housing of the contactor, with the flue gas flowing through the interior and the scrubbing liquid flowing in reverse flow in the outer fiber space. Because the membrane fibers that are used are made of polypropylene and are therefore hydrophobic, the membrane pores are wetted by the gas phase. This means that the membrane material comes in contact with the liquid phase only on the membrane surface. The mass transport of the carbon dioxide takes place based on diffusion through the membrane pores. The selectivity is determined by the solubility of the respective gas in the scrubbing liquid. The advantage of a membrane contactor is the large volume-related mass transfer surface area, which is a multiple of the mass transfer surface area of conventional contact apparatuses [1].

As an alternative to chemical absorption, CO₂ separation by means of selective CO₂ membranes is also being considered for post-combustion processes. For this purpose, polymer membranes, such as mixtures made of polyvinyl alcohols (PVA) and polyvinylamines (PVAm), for example, or organic-inorganic hybrid membranes as well as ceramic membranes, are being developed to separate CO₂ and N₂ at temperatures up to 200° C. Currently, a plastic membrane is also being explored, in which the membrane not only acts as a filter, but also guides the CO₂ in the form of bicarbonate through the membrane using a solvent that is fixed to the membrane.

The disadvantage in the aforementioned examples, however, is that, in the case of chemical absorption, the energy expenditure, in the form of steam withdrawn from the low-pressure steam turbine, for regenerating the absorbent results in a significant decrease in the gross and net electrical efficiency of the power plant, depending on the power plant and the decarbonization plant. According to [2], this decrease can amount to as much as 15 percentage points of the electrical power of a coal-fired power plant.

In the case of CO₂ separation by means of a membrane, in particular a ceramic membrane, the energy expenditure for assuring the driving force that is required for the CO₂ transport through the membrane results in a considerable decrease in the net electrical efficiency of the power plant. Moreover, the degree of CO₂ separation is significantly less than 90% at the defined CO₂ purity level, depending on the membrane selectivity. For example, at a CO₂/N₂ selectivity of 40:1 and a CO₂ purity of 80% by volume, the CO₂ separation of a single-stage polymer membrane is only 50% [3].

Problem and Solution

It is an object of the invention to provide a method which allows for an effective reduction of CO₂ emissions from the waste gases of combustion plants in a simple and cost-effective manner. It is a further object of the invention to provide a suitable device for performing the method mentioned above.

The objects of the invention are achieved by a method according to the main claim and by a device comprising the collective characteristics of the additional independent claim. Advantageous embodiments are apparent from the dependent claims referring back to these claims.

Subject Matter of the Invention

The invention relates to a method for reducing CO₂ emissions from the waste gases of combustion plants, and particularly from flue gases of energy conversion plants, using a combination of methods comprising CO₂-selective membranes and chemical absorption. For this purpose, the method according to the invention comprises the characteristics of claim 1.

The invention further relates to a device that is suitable for performing this method and that is apparent from the characteristics of the independent claims.

Hereafter, a combustion plant shall be understood as any plant in which a gaseous, liquid and/or solid fuel is oxidized so as to use the heat generated. These include, for example, gas burners operated with natural gas, liquefied petroleum gas, city gas, or landfill gas, oil burners operated, for example, with crude oil, heating oil or alcohols, as well as grate firing for clumped fuels, such as gassy coal or wood chips, fluidized bed combustion processes or coal dust firing.

The gaseous combustion product that develops during the industrial combustion of fuels is referred to as flue gas. Flue gases generally comprise solid, liquid and/or gaseous impurities such as, for example, nitrogen, carbon dioxide, sulfur dioxide, nitrogen oxide and water vapor in the form of gases, solid particles such as fly ash and soot, and optionally carbon monoxide or hydrogen.

The idea of the invention is based on performing CO₂ separation that is improved and more effective as compared to the prior art, during energy conversion, by combining a CO₂-selective membrane and an additional CO₂ separation step, for example chemical absorption.

According to the invention, in a first step, partial CO₂ separation is carried out by way of a CO₂-selective membrane, or a CO₂-selective membrane module, which hereafter is also referred to as a membrane unit. For this purpose, the manner in which the CO₂ partial pressure difference is generated is immaterial or variable within wide ranges (pressure on the feed side, vacuum on the permeate side, flushing medium, or a combination of two or more of these options). The enriched CO₂ is captured on the permeate side of the membrane module. The concentration of CO₂ in the permeate is usually higher than in the flue gas (feed) that is supplied to the membrane module. The waste gas from the membrane module (retentate) contains less CO₂ than before, so that the CO₂ partial pressure/CO₂ concentration is lower than on the feed side. In general, it is not difficult to achieve CO₂ depletion in the retentate of at least 30%, advantageously as much as at least 40%, and particularly advantageously as much as in excess of 50%, of the CO₂ starting concentration in the flue gas.

In the second step, the waste gas from the membrane module (retentate) is supplied to an additional CO₂ separation unit, which separates additional CO₂ from the waste gas. For this purpose, absorption and/or adsorption methods are notably suitable. If an absorption step follows, reference is made hereafter to a downstream absorption unit.

A characteristic of chemical and/or physical absorption is that the CO₂ concentration in the waste gas that is supplied, and which is to be treated, has no, or only little, influence on CO₂ separation from the supplied waste gas. The fact that some of the CO₂ was already separated by the preceding membrane module does not, therefore, negatively impact the effectiveness of this separation step in any way. If less CO₂ is separated in the absorption unit, advantageously less energy is required for the desorption of CO₂ and less absorbent is required in the absorption unit.

The basic idea of the present invention consists in suitably combining the advantages of both CO₂ separation methods so that effective, yet energy-saving, separation can be performed, which lowers the efficiency of the entire combustion plant only moderately. “Moderately” within the scope of the invention shall mean that, in total, no more than 10 percentage points of the electrical power of the power plant, and more particularly no more than 5 percentage points, is required for decarbonization.

The advantages of combining the methods of a CO₂-selective membrane (membrane unit) and an additional separation step, notably absorption (absorption unit), are summarized as follows:

1) lower total energy expenditure as compared to chemical absorption alone, at the same degree of CO₂ separation;

2) greater purity of enriched CO₂ at a high degree of separation as compared to a CO₂-selective membrane alone, with the same degree of separation;

3) decreased energy expenditure for regenerating the absorbent in the absorption unit when the waste heat from the membrane unit is utilized;

4) reductions in the amount of absorbent, the desorber and the peripheral systems of the absorption unit, as compared to chemical absorption alone, because the CO₂ flow to be separated is reduced as a result of the upstream membrane unit.

Separation by means of a CO₂-selective membrane is highly dependent on the driving force of the CO₂ diffusion through the membrane and generally decreases in terms of effectiveness with lower concentrations of CO₂ in the flue gas.

Separation by means of a selective membrane is thus used at a point at which it takes little effort, in terms of the production of the partial pressure difference, to separate a portion of the CO₂ from the flue gas. Additional CO₂ separation by means of a membrane could be achieved at this point only by way of a greater partial pressure difference, which would involve either large volume flows of flue gas, large quantities of flushing gas on the permeate side and/or particularly high complexity in the production of the partial pressure difference. Disadvantageously, all this could only be achieved with high energy expenditure. Moreover, at high degrees of separation, further CO₂ separation would lower the CO₂ purity in the product.

At this point, according to the invention, absorptive separation is thus provided downstream. This offers the advantage that the degree of CO₂ separation in this absorptive method step is substantially independent of the concentration of the supplied gas, but rather depends primarily on the parameters of the absorbent and the method. It is thus advantageously possible to remove additional CO₂ from the flue gas that has already been highly depleted of CO₂, using simple means. Because both the concentration of CO₂ and the absolute flue gas quantity are lower downstream of the membrane unit, than without such a membrane unit, the subsequent absorption can likewise advantageously be designed in a reduced manner. This reduces not only the quantity of the absorbent, but in particular the amount of energy required for desorption of the absorbed CO₂ from the scrubbing liquid.

This combined method according to the invention for CO₂ separation from the flue gas of a combustion plant thus advantageously makes it possible to achieve degrees of separation as high as 90%, without reducing efficiency by more than 8 percentage points as a result of this separation.

Within the scope of the invention, the membrane unit may comprise one or more membranes in any arbitrary interconnection. Moreover, the required driving force can be achieved by pressure on the feed side, a vacuum on the permeate side, flushing medium, or a corresponding combination of two or more of these options. Moreover, depending on the condition of the flue gas or of the enriched CO₂, the membrane unit may comprise additional components.

The combination of methods can advantageously be carried out in various positions in the existing flue gas scrubbing process along the flue gas duct. Advantageously, other flue gas scrubbing and conditioning methods may be provided between the two steps in the described combination of methods.

So as to produce the CO₂ flow in a defined quality, the CO₂ flows generated by means of the two method steps can advantageously be combined at various proportions.

SPECIFIC DESCRIPTION

The invention will be described in more detail hereafter with reference to exemplary embodiments, without thereby limiting the scope of protection. The person skilled in the relevant art will recognize analogous modifications as part of the invention.

In FIGS. 1 and 2, the following meanings apply:

a1) flue gas,

a2) partially CO₂-depleted flue gas,

b) CO₂-depleted waste gas,

c) CO₂-enriched scrubbing liquid,

d) regenerated scrubbing liquid,

e1), e2) separated, gaseous CO₂,

f) cooling circuit for separated CO₂, or heating circuit for regenerating the absorbent,

g) CO₂-enriched permeate.

FIG. 1 shows a diagram of an energy conversion process, which is an example of an energy generation process in a coal-fired power plant comprising CO₂ separation from the flue gas (decarbonization) by means of chemical scrubbing (for example, 30% aqueous solution of monoethanolamine) subsequent to flue gas scrubbing according to the present state of the art. Flue gas scrubbing of a large-scale combustion plant fired with solid fuel, corresponding to the present state of the art, comprises nitrogen oxide reduction, dedusting, and desulfurization, in this order, in the case of a coal-fired heating.

Approximately 90% of the CO₂ contained in the flue gas (1 a) is separated at a purity of 99% by volume (e1)) by chemical scrubbing. In a defined design, 6.3 percentage points of the electrical power of the coal-fired power plant is required to regenerate the absorbent, and for the electrical consumption of the decarbonization plant.

FIG. 2 shows a diagram of an energy conversion process using the combination of methods according to the invention comprising the CO₂-selective membrane (membrane unit) and chemical scrubbing (absorber unit).

Using the CO₂-selective polymer membrane having a CO₂/N₂ selectivity of 40:1, 50% of the CO₂ contained in the feed gas is separated in the membrane unit. The enriched CO₂ (g) has a purity of 80% by volume. In the example, the CO₂ partial pressure difference on the feed and permeate sides of the membrane is generated by means of a vacuum pump, the power requirement of which is approximately 2 percentage points of the electrical power of the coal-fired power plant.

Subsequently, the retentate (a2) from the membrane module is supplied to the CO₂-selective chemical scrubbing process (30% aqueous solution of monoethanolamine). By forming the chemical bonds between the monoethanolamine and CO₂ in the absorber, CO₂ is removed from the flue gas (b). After heating the CO₂-enriched, 30% aqueous solution of monoethanolamine by way of steam from the low-pressure turbine in the desorber, chemical bonds between the monoethanolamine and CO₂ are destroyed, so that the gaseous CO₂ (e1) can be captured and conditioned further. The depleted aqueous solution of monoethanolamine (d) is available again for subsequent use after conditioning. After conditioning the separated, gaseous CO₂, it has a purity of 99% by volume. It is assumed that 90% of the CO₂ that is supplied to chemical scrubbing is separated by means of the chemical scrubbing process. However, because the CO₂ mass flow separated by means of chemical scrubbing now accounts for less than half the CO₂ originally present in the flue gas, the regeneration of the absorbent and the electrical consumption of the decarbonization plant require approximately 3 percentage points of the electrical power of the coal-fired power plant. The entire energy requirement for decarbonization is now advantageously only 5 percentage points of the electrical power of the coal-fired power plant.

When using the waste heat of the vacuum pump in the absorption unit for regenerating the absorbent, the energy requirement of the absorption unit decreases to approximately 2.4 percentage points of the electrical power of the coal-fired power plant, so that the entire energy requirement for decarbonization is now only approximately 4.4% of the electrical power of the coal-fired power plant.

The enriched CO₂ from the membrane module (e2) can optionally be mixed with the CO₂ from the chemical scrubbing process (e1), so that, as a result of the entire CO₂ separation, 95% of the entire CO₂ is separated from the flue gas at a purity of approximately 90% by volume, which is sufficient for further transport and storage according to the literature. In total, a CO₂ separation degree of at least 80%, advantageously even 90%, and particularly advantageously more than 95%, of the CO₂ from the flue gas is possible.

Literature cited in this application:

-   [1] Bernhard Epp, Hans Fahlenkamp and Christina Stankewitz, Chemie     Ingenieur Technik [Chemical Engineer Technology] 2008, 80, No. 10. -   [2] G. Göttlicher (1999) Energetik der Kohlendioxidrückhaltung in     Kraftwerken [Energetics of carbon dioxide retention in power     plants], Düsseldorf, VDI Verlag. -   [3] L. Zhao, E. Riensche, R. Menzer, L. Blum, D. Stolten (2008) A     parametric study of CO₂/N₂ gas separation membrane processes for     post-combustion capture, Journal of Membrane Science, 325, 284-294. 

1. A method for separating carbon dioxide from a flue gas, comprising supplying the flue gas to a membrane unit, comprising at least one membrane module having a CO₂-selective membrane, with a portion of the CO₂ being separated in the module from the flue gas as enriched CO₂ permeate, and supplying the flue gas thus depleted (retentate) to at least one additional CO₂ separating unit, in which a portion of the CO₂ is separated from the flue gas by an adsorbent or absorbent.
 2. The method according to claim 1, wherein a portion of the CO₂ is separated from the flue gas by a plurality of membrane modules, each having a CO₂-selective membrane.
 3. A method according to claim 1, wherein a partial pressure difference is produced inside a membrane module either by pressure on the feed side, or by a vacuum on the permeate side, or by a flushing medium, or by the combination of two or more of these options.
 4. A method according to claim 1, wherein the flue gas thus depleted is supplied to an absorption unit of an additional CO₂ separating unit.
 5. A method according to claim 4, wherein the CO₂ is removed from the flue gas in the absorption unit by means of a regenerative absorbent.
 6. A method according to claim 1, wherein the CO₂ is removed from the flue gas by means of an absorption scrubber or a membrane contactor.
 7. A method according to claim 1, wherein the CO₂ is removed from the flue gas by way of multistage absorption.
 8. A method according to claim 1, wherein the absorbent is regenerated and the absorbed CO₂ is released again.
 9. A method according to claim 1, wherein the heat of the permeate enriched with CO₂ from at least one membrane module is used to regenerate an absorbent.
 10. A method according to claim 4, wherein the CO₂-enriched substance-flows from one or more membrane units and the absorption unit are combined.
 11. A method according to claim 1, wherein at least 30%, advantageously more than 40%, and particularly advantageously more than 50%, of the CO₂ is separated from the flue gas in the membrane unit.
 12. A method according to claim 1, wherein at least 80%, advantageously more than 90%, and particularly advantageously more than 95% of the CO₂ is separated from the flue gas by combining the membrane unit and an additional CO₂ separating unit.
 13. A device for separating CO₂ from a flue gas, having a membrane unit, comprising at least one membrane module having a CO₂-selective membrane and an additional CO₂ separating unit, comprising an adsorbent or an absorbent.
 14. A device according to claim 13, comprising a plurality of membrane modules, each having a CO₂-selective membrane.
 15. A device according to claim 13, comprising an absorption unit as the additional CO₂ separating unit.
 16. A device according to claim 15, wherein the absorption unit comprises an absorption scrubber or a membrane contactor.
 17. A device according to claim 15, wherein the absorption unit comprises a means for regenerating an absorbent.
 18. A device according to claim 17, wherein a heat exchanger is disposed between the membrane module and the means for regenerating.
 19. A device according to claim 13, comprising a means for combining the CO₂-enriched substance-flows from the membrane unit and the additional CO₂ separating unit. 